
/*public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if(root == null) return list;
        list.add(root.val);

        List<Integer> leftTree = preorderTraversal(root.left);
        list.addAll(leftTree);

        List<Integer> rightTree = preorderTraversal(root.right);
        list.addAll(rightTree);

        return list;
    }
}*/


